Gyrate atrophy (GA) of the choroid and retina is an autosomal recessive eye disorder involving a progressive loss of vision due to chorioretinal degeneration. A variety of ornithine-delta-aminotransferase (OAT) gene mutations have been reported in GA patients and are suspected to be associated with this ocular disease. However, the precise mechanism by which the OAT deficiency and hyperornithinemia lead to the chorioretinal degeneration remains unknown. To elucidate the pathophysiological role of OAT, we are attempting to create OAT-deficient mice by gene targeting via embryonic stem (ES) cells. Toward this ultimate goal, we isolated murine OAT gene to construct OAT targeting replacement vector.